1. Field of the Invention
This invention relates to a novel sintered high-density cubic boron nitride and a process for producing the same.
2. Description of the Prior Art
Sintered cubic boron nitride (hereunder referred to as CBN) has excellent physical properties that suggest its potential use in applications such as a cutting tool, a drawing-die, a grinding wheel, an IC heat sink and a high-frequency semiconductor. However, it is almost impossible to sinter cubic boron nitride thoroughly and for this reason metal or metal substitutes have been used as a binder for the sintered CBN. The type of binder and its content greatly affects the performance of the sintered CBN and the sintering conditions. For example, sintered CBN using cobalt or the like as the binder has a binding phase that comprises a brittle intermetallic compound and is deleterious to the performance of the sintered body. Sintered CBN using heat-resistant and tough ceramics such as TiN and AlN is not free from the defect of being brittle.
Therefore, various efforts have been directed to eliminating the defects of conventional sintered CBN. Since CBN remains stable under high-temperature and pressure conditions as shown in the phase diagram of FIG. 1, it must be sintered in such a stable region. However, experiments conducted by the inventors have shown even when CBN is sintered at 65 Kb and 1,750.degree. C. using only a ceramic binder such as TiN or TiN-AlN its density ratio increases to just about 98% and cracks developed in the sintered body. A binder comprising TiN-Si provides a higher density ratio but the resulting sintered compact has cracks in it. (The term "density ratio" is the ratio of the density of the sintered product actually obtained to the density calculated from the theoretical density of each component in the composition). Such phenomena could be explained by the low sinterability of the ceramic that forms the binding phase and insufficient affinity of CBN for the binding phase. Thinking that the improvement of these points may result in the ability to produce a good sintered body under less severe conditions, the inventors have found that in the presence of a specific cermet, a sintered CBN having excellent properties can be obtained under pressures lower than those conventionally used without transformation to hexagonal boron nitride.